The present invention relates to an optical system for an optical pick-up that is capable of using a plurality of kinds of optical discs whose thicknesses of cover layers are different from each other. Particularly, the present invention relates to an optical system whose objective lens has a diffractive lens structure.
The optical disc has an information layer, on which digital information is recorded, and a transparent cover layer, which covers the information layer.
There are several types of optical discs whose cover layers have different thickness. For example, the thickness of the cover layer of a compact disc (CD) or a CD-recordable (CD-R) is 1.2 mm, and that of a digital versatile disc (DVD) is 0.6 mm.
Due to the difference in thickness of the cover layers, when an optical disc placed on a turn table of a disc drive is replaced with another type optical disc, the distance between the optical pick-up and the information layer changes.
An optical pick-up typically includes a light source for emitting a laser beam, an objective lens for converging the laser beam onto an information layer of an optical disc through a cover layer thereof, and a light detector for receiving a reflected laser beam from the optical disc to detect signals. In some optical pick-ups compatible with a plurality of types of optical discs, a plurality of independent optical systems for respective types of optical discs are provided. However, for a compact design of an optical pickup, it is required that the number of optical components is reduced, and therefore, it is desirable to use a common objective lens which can be used for a plurality of types of optical discs.
When the common objective lens is employed, by moving the objective lens in the optical axis direction, a paraxial beam waist (a beam spot) position can be changed so as to meet the thickness of the cover layer. However, the difference in thickness of the cover layer changes spherical aberration. If only the objective lens is moved when the disc is replaced with the different type disc, wavefront aberration of the laser beam becomes large. For instance, when the objective lens, which is designed to minimize the spherical aberration for a DVD, is used for readout of the information from a CD, the spherical aberration becomes too large to reproduce the information even if the objective lens is moved to place the beam spot on the information layer of the CD.
A method has been known in which the degree of convergence/divergence of a beam incident on the objective lens is changed according to the type of the optical disc (CD or DVD), thereby correcting the spherical aberration caused by the difference in thickness of the cover layer. In such a case, a distance from a light emission point to the objective lens for a CD should be shorter than a distance from the light emission point to the objective lens for a DVD. With this configuration, the laser beam for the CD is incident on the objective lens as a divergent beam and the laser beam for a DVD is incident on the objective lens as a parallel-light beam.
It has also been known to form a diffractive lens structure on a surface of an objective lens to compensate for a change of spherical aberration due to the difference of the thickness of the cover layers. Since the diffractive lens structure exhibits a wavelength dependence, a change of spherical aberration due to a difference in thickness of the cover layers can be compensated for by changing a wavelength of the laser beam.
However, when the degree of convergence/divergence is varied to correct spherical aberration caused by the difference in thickness of the cover layer, off-axis aberration becomes larger for at least one of a CD and a DVD. Accordingly, if it becomes necessary to use an off-axis beam due to a lens shift in tracking operation and/or an angle error or a position error in assembling, relatively large aberration mainly including coma is generated.
If the spherical aberration is corrected by employing the diffractive lens structure without changing divergence/convergence of the laser beam incident on the objective lens, a back focus remains substantially unchanged regardless of the types of the optical discs. In this case, a working distance for a CD or a CD-R, which has a thicker cover layer, is shortened.
The present invention is advantageous in that there is provided an optical system for an optical pick-up, which is capable of recording/reproducing data to/from a plurality of kinds of optical discs having cover layers of different thicknesses, such as a CD and a DVD, that exhibits excellent off-axis performance and keeps a sufficient working distance even for an optical disc having a thicker cover layer such as a CD or a CD-R.
According to an aspect of the invention, there is provided an optical system for an optical pick-up, the optical system is provided with a light source portion that selectively emits a first laser beam and a second laser beam whose wavelength is longer than that of the first laser beam, and an objective lens for converging the first laser beam onto an information layer of a first optical disc through a cover layer thereof and for converging the second laser beam onto an information layer of a second optical disc through a cover layer thereof. The second optical disc has lower recording density and a thicker cover layer than the first optical disc.
Further, the light source portion is designed such that the divergence of the second laser beam incident on the objective lens is larger than that of the first laser beam, correcting one part of spherical aberration caused by a difference in thickness of the cover layer.
At least one surface of the objective lens is divided into a central area through which a laser beam of low NA, which is necessary and sufficient for the second optical disc, passes and a peripheral area through which a laser beam of high NA, which is necessary only for the first optical disc, passes. A diffractive lens structure that changes spherical aberration in accordance with wavelength is formed within at least the central area so that the residual part of the spherical aberration caused by the difference in thickness of the cover layer is corrected by switching the wavelength of the laser beam.
With this construction, one part of the spherical aberration caused by the difference in thickness of the cover layer is corrected by changing the divergence of the laser beam, i.e., by changing the imaging magnification of the objective lens, and the residual spherical aberration is corrected by the wavelength dependence of the diffractive lens structure. Therefore, the spherical aberration can be corrected while keeping off-axis performance well. Further, the divergence of the second laser beam is larger than that of the first laser beam, which increases a back focus when the second optical disc is used, reducing a variation between the working distance for the first optical disc and the working distance for the second optical disc.
The objective lens preferably satisfies the condition (1):
0.01 less than xcex94SAxc2x7(fl/xcfx862)2 less than 0.03xe2x80x83xe2x80x83(1)
where
xcex94SA is the residual spherical aberration (unit: mm) at the edge of the effective diameter in the central area when the laser beam whose wavelength is equal to the first laser beam and whose divergence is equal to the second laser beam is incident on the second optical disc;
fl is a focal length (unit: mm) of the objective lens at the wavelength of the first laser beam, and
xcfx862 is the maximum diameter (unit: mm) of the central area.
An additional optical path length added by the diffractive lens structure is expressed by the following optical path difference function "PHgr"(h):
xe2x80x83"PHgr"(h)=(P2h2+P4h4+P6h6+ . . . )xc3x97kxc3x97xcex
where
P2, P4 and P6 are coefficients of second, fourth and sixth orders;
h is a height from the optical axis;
k is a diffraction order; and
xcex is a working wavelength.
It is preferable that the following conditions (2) and (3) are satisfied;
xe2x88x92180 less than (P4xc2x7fl4)/k less than xe2x88x9260xe2x80x83xe2x80x83(2).
0.02 less than (m1xe2x88x92m2)xc2x7fl less than 0.15xe2x80x83xe2x80x83(3)
where
m1 is the imaging magnification of the objective lens when the first optical disc is used; and
m2 is the imaging magnification of the objective lens when the second optical disc is used.